The present invention relates to a new thermosetting transparent coating material. The present invention also relates to a new process for preparing a thermosetting transparent coating material. The present invention additionally relates to the use of the new thermosetting transparent coating material for producing coatings by the coil coating process.
Modern automobiles, especially top-class automobiles, have multicoat color and/or effect paint systems. These systems, as is known, comprise an electrocoat, a surfacer coat, antistonechip primer coat or functional coat, a color and/or effect basecoat, and a clearcoat. The multicoat paint systems are produced by means of what are termed wet-on-wet techniques, in which a clearcoat film is applied to a dried but not yet cured basecoat film and then at least basecoat film and clearcoat film are conjointly thermally cured. This process may also include the production of the electrocoat and of the surfacer, antistonechip primer or functional coat.
The multicoat color and/or effect paint systems are required to have what has been termed automobile quality. According to European Patent EP 0 352 298 B1, page 15, line 42, to page 17, line 14 this means that the multicoat paint systems in question score highly for
(1) gloss,
(2) distinctiveness of image (DOI, i.e., of the reflected image),
(3) hiding power and uniformity thereof,
(4) dry film thickness uniformity,
(5) gasoline resistance,
(6) solvent resistance,
(7) acid resistance,
(8) hardness,
(9) abrasion resistance,
(10) scratch resistance,
(11) impact strength,
(12) intercoat and substrate adhesion, and
(13) weathering and UV stability.
Other important technological properties include
(14) high resistance to condensation,
(15) absence of any propensity toward blushing, and
(16) high stability toward tree resin and bird droppings.
The clearcoats in particular are marked by such essential technological properties as
(1) gloss,
(2) distinctiveness of image (DOI, i.e, of the reflected image),
(5) gasoline resistance,
(6) solvent resistance,
(7) acid resistance,
(8) hardness,
(9) abrasion resistance,
(10) scratch resistance,
(13) weathering and UV stability,
(14) high resistance to condensation,
(15) resistance to blushing and
(16) stability toward tree resin and bird droppings.
The quality of the clearcoats is therefore subject to particularly stringent requirements.
However, particular requirements are also imposed on the technological properties of the clearcoat materials from which these clearcoats are produced. To start with they must provide the clearcoats in the requisite quality without problems and with outstanding reproducibility, and they must be preparable in a simple and outstandingly reproducible way.
The production of multicoat paint systems by the process described above is carried out on the line at the automaker's plant and involves a high level of complexity in terms of environmental considerations, process engineering, and apparatus, representing a significant portion of the production costs.
The automobile industry is therefore at pains to replace bodywork components, such as hood, trunk or doors, by components which are already painted in the vehicle color.
A most essential requirement for such a process, however, is that corresponding coated coils can be produced by means of the coil coating process and, in the painted state, can be brought into the desired form by means of shaping techniques, in particular by deep drawing, by the automaker or by the manufacturer of exterior mounted components.
Coil coating is the term used for a special form of roller coating (Römpp Lexikon Lacke und Druckfarben, Georg Thieme Verlag, Stuttgart, New York, 1998, page 617, “Roller Coating”) and also, occasionally, the spray coating and flow coating of metal strips (“coils”) with liquid coating materials. It is a continuous process, e.g. all operations such as cleaning, pretreatment, painting, and curing, etc., are conducted in one operation in one installation. Schematically, the steps involved in coil coating are as follows: The cleaning and degreasing of the coil are followed by a multistage chemical pretreatment with subsequent passivation, rinsing, and drying. Cooling is followed by the application of the liquid coating material to one or two sides using two or three rolls, usually by the reverse roller coating technique. After a very short evaporation time, the applied coat is thermally cured at temperatures from 180 to 260° C. for from 20 to 60 s. For the production of a multicoat paint system, application and curing are repeated. The speeds of coil coating lines are up to 250 m/min (Römpp Lexikon Lacke und Druckfarben, Georg Thieme Verlag, Stuttgart, New York, 1998, page 55, “Coil Coating”).
The reason why attempts to transfer this inherently advantageous process to the automobile sector have so far essentially failed is that the clearcoat materials employed to date do not have the required automobile quality, even before deformation. In particular it is not possible to reproduce the shades and optical effects the automobile industry requires and the requisite gloss and overall appearance, since the coated coils have roller textures which are visible with the naked eye.
In coil coating processes, moreover, it is common to use clearcoat materials based on polyester binders and melamine resin crosslinking agents. Their good wetting and good leveling give these clearcoat materials good processing properties. Although the coatings produced from them are very flexible and lend themselves very well to deformation, they fail to meet the required automobile quality in terms of chemical resistance and weathering stability.
Clearcoat materials based on (meth)acrylate (co)polymer binders and blocked polyisocyanate crosslinking agents do usually give clearcoats of high automobile quality, particularly as regards gloss, hardness, and chemical resistance. Optimum leveling of the clearcoat materials and optimum weathering stability and distinctiveness of image can be achieved only by means of additives. The clearcoat materials cannot be used in the coil coating process owing to the relatively low flexibility of the clearcoats.
In order to utilize the advantages of the clearcoat materials based on (meth)acrylate (co)polymers and blocked polyisocyanates, and those of the clearcoats produced from them, in the coil coating process as well, German patent application DE 100 59 853 A 1 proposes a clearcoat material comprising    (1) 10 to 70% by weight of a nonaqueous solution of an acrylate-based polymer having a hydroxyl number of between 100 and 250,    (2) 10 to 70% by weight of a nonaqueous solution of a fluorine-modified polymer having a glass transition temperature of between 20 and 40° C., and    (3) 20 to 60% by weight of at least one blocked aliphatic or cycloaliphatic polyisocyanate, andthe weight ratio of components (1) to components (2) being not more than 1 and the sum of components (1), (2), and (3) being 100%, based on the binder content.
This known coating material does give transparent coatings which are of automobile quality in terms of gloss, hardness, flexibility, and chemical resistance. Owing to the comparatively poor wetting and the comparatively poor leveling, however, the known coating material does not have good processing properties. The coatings produced from them have the substantial disadvantage of including considerable amounts of fluorine compounds. Fluorine, however, is known to cause considerable problems in the recycling of old cars, especially when the bodywork is made of aluminum. Moreover, the coatings are difficult to overcoat, which causes considerable problems for automotive refinish. And, not least, the fluorine compounds are comparatively expensive, thus making the coating material and the coatings produced therefrom less attractive from an economic standpoint.
It is an object of the present invention to provide a new thermosetting transparent coating material which no longer has the disadvantages of the prior art but is instead free from organic fluorine compounds, so that the coated substrates produced from it, especially the aluminum-based automobile bodies, can be recycled without problems. The new thermosetting transparent coating material ought also to have particularly good wetting and particularly good leveling and therefore to have particularly good processing properties. In addition it should be able to be prepared using comparatively inexpensive constituents, so that it is advantageous from the standpoint not only of technology but also of economics.
The new thermosetting transparent coating material should be simple to prepare and lend itself advantageously to use in the coil coating process, to give transparent, in particular clear, coatings on coils, possessing the automobile quality, particularly in respect of gloss, surface smoothness, distinctiveness of image, hardness, weathering stability, and chemical resistance.
The new transparent, in particular clear, coatings ought also to have very good overcoatability, so that they can be refinished to very good effect.
After their processing by shaping, the precoated coils ought to continue to have the automobile quality, so that the coated shaped parts, as exterior mounted components, can be installed in correspondingly coated, larger units, especially correspondingly coated automobile bodies, without deleterious manifestation of a visual difference, particularly in the region of edges which abut one another.